The present invention relates to a shift control system for a continuously variable traction roller transmission.
One of the previously proposed continuously variable traction roller transmissions is disclosed, for example, in JP-A 2-163567. Such a continuously variable transmission may use a shift control system as shown in FIGS. 64-66.
Referring to FIGS. 64-66, 1 generally designates a power roller which is interposed between input and output toroidal disks 2, 3 disposed axially perpendicular thereto. An opposed surface of each of the input and output toroidal disks 2, 3 is a toroidal surface, whereas a periphery of the power roller 1 is a spherical surface which is contact therewith. Thus, torque to the input toroidal disk 2 is transmitted to the output toroidal disk 3 via rotation of the power roller 1. By slantly rotating the power roller 1 about a slant rotation axis O.sub.2 perpendicular to a rotation axis O.sub.1 thereof to vary a position of contact with the input and output disks 2, 3, the input/output rotation ratio or speed ratio can be changed steplessly.
While the power roller 1 occupies a position to put the rotation axis O.sub.1 at the same level as a rotation axis O.sub.3 of the input and output corn disks 2, 3, it preserves the inclination or speed ratio at the same value, whereas when the power roller 1 is moved in the direction of the slant rotation axis O.sub.2 to put the rotation axis O.sub.1 offset from the rotation axis O.sub.3 of the input and output disks 2, 3, it slantly rotates by itself in the direction corresponding to the offset direction to vary the inclination. From the foregoing, when controlling this continuously variable transmission, the following shift control method is adopted:
A trunnion 4 for rotatably supporting the power roller 1 is not only rotatable about the slant rotation axis O.sub.2, but movable in the direction of slant rotation axis O.sub.2. This displacement is produced by a plurality of servo pistons 5 which are in turn controlled in stroke by a shift control valve 6. For that purpose, the shift control valve 6 has an input port 6a to which a line pressure is supplied, and output ports 6b, 6c connected to chambers on both sides of each servo piston 5, respectively. Additionally, the shift control valve 6 receives a speed ratio command from a pinion 7 driven by a stepper motor (not shown). An input sleeve valve body 8 is stroked in accordance with this command, so that a relative position thereof with respect to a feedback spool valve body 9 is changed from a balance position as indicated in FIG. 65. Thus, the shift control valve 6 supplies in response to a new speed ratio command a line pressure of the input port 6a from the one output port 6b to the corresponding side of each servo piston 5, and drains the opposite side thereof from the other output port 6c, putting the power roller 1 offset in the corresponding direction and with respect to the input and output toroidal disks 2, 3. Under such offset conditions, the power roller 1 changes the inclination by itself in the direction corresponding to the speed ratio command.
The displacement or offset to the slant rotation axis O.sub.2 and slant rotation of the power roller 1 are transmitted, via the displacement to the slant rotation axis O.sub.2 and rotation of a precess cam 10 connected to the power roller 1, to a link 11 which is in contact with a cam face 10a thereof, and are fed back to the feedback spool valve body 9 of the shift control valve 6 via rotation of the link 11 about a shaft 12. The link 11 is an L-shaped lever, and has one lever arm in contact with the cam face 10a of the precess cam 10, and the other lever arm abutting on the feedback spool valve body 9 via an adjust screw 13 engaged therewith. This feedback enables the feedback spool valve body 9 to follow the displacement of the input sleeve valve body 8 in response to the speed ratio command so as to regain the initial relative position with respect to the input sleeve valve body 8. Accordingly, as soon as the power roller 1 has the inclination corresponding to the speed ratio command, the shift control valve 6 puts at zero the offset of the power roller 1 via the servo pistons 5, preserving the inclination or speed ratio.
With such a shift control system for a continuously variable traction roller transmission, it is preferable that a response lag is small which is produced between receiving of the speed ratio modifier command and achievement of the corresponding inclination of the power roller 1, and that the speed ratio converges quickly on a speed ratio command value with small frequency of vibrations or hunting which is produced until the inclination of the power roller 1 or speed ratio settles to the command value during a transition period of modification of the speed ratio.
One of the factors affecting such shift control performances is the type of the shift control valve 6. When using an overlap type shift control valve, i.e., the shift control valve wherein valve port parts of the input sleeve valve body 8 and the feedback spool valve body 9 overlap or engage each other in a balance state thereof after completion of the shift, relatively frequent hunting is produced. On the other hand, when using an underlap type shift control valve, i.e., the shift control valve wherein the valve port parts of the two valve bodies 8, 9 do not engage each other to keep valve ports open even in the balance state thereof, the response lag is larger, so that the shift control is practically difficult to carry out.
On the other hand, referring to FIG. 65, there is a just lap type shift control valve, i.e., the shift control valve wherein the valve port parts of the two valve bodies 8, 9 just close the valve ports in the balance state thereof. However, although not as considerable as compared with the underlap type shift control valve, the just lap type shift control valve is not free from a constant leakage of hydraulic fluid out of the valve ports even in the balance state of the shift control valve, thereby enlarging energy loss of an engine for driving an oil pump.
It is, therefore, an object of the present invention to provide a shift control system for a continuously variable traction roller transmission including an overlap type shift control valve which is free from not only the problem of hunting, but also response lag.